Why did Lucretius believe that atoms cannot be directly perceived by the senses?

In his philosophical work De rerum natura (On the Nature of Things), Lucretius explained that atoms are inherently imperceptible to the senses because they lack dischargeable parts.1

While the senses are considered infallible in Lucretian philosophy, they function by perceiving the properties and accidents of bodies—often through the discharge of representative films from the surface of objects that strike the eye or other sensory organs. Because individual atoms are the fundamental, indivisible building blocks of matter, they do not possess these dischargeable parts and therefore cannot be directly detected by human perception. Consequently, while we can observe the effects of atomic movement and combinations, the existence of the atoms themselves must be inferred through reason.1